The non-linear error is an important performance indication for a measuring instrument and directly correlates with the output result of the instrument. In the technical field of the precise metrology, the measuring instrument is generally calibrated by a zero calibration and a known optical amount of standard sample calibration; and then calculates the measurement results in accordance with the ratio of the measurement signals of measured sample to standard sample. However, during this process, the non-linear error of the measuring instrument directly causes the indeterminacy of the measurement results. Therefore, in the field of precise metrology, there is a need to precisely measure and calibrate the non-linear error of the instrument for ensuring the accuracy of the measurement results.
Conventional measurement methods for the non-linear error of an optical measuring instrument are as follows:
G. L. Klein has proposed a relatively common method for measuring non-linear error in U.S. Pat. No. 4,059,357 “Densitometer calibrated reference standard”, in which the non-linear error of the instrument may be measured and calibrated by using a group of samples with known parameters. This method belongs to a relative measurement method.
H. Bennett has proposed a highly precise measurement method for the non-linear error by rotating a polarizer to attenuate a light source, in an article “Accurate method for determining photometric linearity”. This method is not dependent on the accuracy of the nominal value of a sample, but it utilizes a precise mechanism to cause high performance polarizer to rotate, and achieves a precise attenuation of the intensity of the light source. This method belongs to an absolute measurement method for non-linear error. The measurement accuracy of the non-linear error would be restricted by the degree of polarization of the polarizer and the accuracy of the mechanically rotational angle. The measurement accuracy for the non-linearity of a detector can reach approximately 0.1%.
C. Sanders has proposed a double-apertures superposition method in an article “A photocell linearity tester”. The principle of the method is as follow: the light source may be divided into two parts by a double-apertures; the detector of the optical measuring instrument may respond to light sources Φ1 and Φ2 as ρ1 and ρ2, respectively; and output ρall as respond to Φ1+Φ2; if ρ1+ρ2≠ρall, then the optical measuring instrument has non-linear error. This method belongs to an absolute measurement method for non-linear error.
Non-linear error of the optical measuring instrument may be caused by many factors such as detector, integrating sphere, light path and so on. The measurement method for non-linear error, proposed by G. L. Klein, uses the samples with known parameters to perform the measurement of non-linear error. This method may comprehensively consider non-linear error for the measuring results of the whole optical measuring instrument, but its accuracy would be restricted by the indeterminacy of the nominal value, and thus it is difficult for improving the measurement accuracy of non-linear error. The polarizing method proposed by H. Bennett and the aperture method proposed by C. Sanders may change the reflectance or the colorimetric values in different areas of a sample port of an instrument by attenuating the intensity of the light source. Although the polarizing method and the aperture method may have a relatively high measurement precision for non-linear error of the optical measuring instrument, taking the reflectance measuring instrument as an example, the reflectance of the element to be measured may influence the transmission efficiency and light distribution of the integrating sphere in the light path, which may bring non-linear error. For such instruments, since the double-apertures method can only modulate the light source, it could only achieve an absolute measurement for the non-linear error of a detector in an optical measuring instrument, and could not achieve a non-linear measurement for final output results of an optical measuring instrument.
To sum up, a technical problem required to address urgently by those skilled in the art is how to achieve a non-linear error measurement for final outputting results of an optical measuring instrument and ensure the accuracy of the non-linear error measurement.